Approximately 700.000 tons of instant coffee is produced each year. The largest production cost is the price of the green coffee beans used. Thus, there is an incentive in reducing the consumption of green beans from today's typical value of approx. 2.4 kg green beans/kg of dry instant powder.
Typically, 2.4 kg green beans yield around 1.0 kg final product. The losses stem from roasting and extraction, where typically 0.4 kg is removed by roasting, and the extraction yields about 50% of the remainder. The level of yield corresponds to the sum of the naturally soluble components of typically approx. 30% plus an additional approx. 20% solubilised components recovered using advanced techniques. The state of the art yield found in the literature is 54-60%.
Traditional solubilisation is made by breakdown using wet, thermal hydrolysis at elevated temperatures. One advanced method for increasing the yield is to increase the wet hydrolysis temperature to 180° C. or above. While higher temperature will increase the amount of hydrolysis products, the high temperature will also contribute to the production of off-flavours, toxic substances, and degradation of solubilised components.
Hence, there has been a need for carrying out the solubilisation by lower temperature methods. Enzymatic breakdown of the various polymeric carbohydrates is an interesting possibility. Also the cost of the enzymes used and heat treatment has to be taken into account in the overall economic evaluation.
Advanced methods for increasing the yield and reducing process complications have been suggested in the prior art. Thus, GB 1.200.700 discloses the use of additional water-soluble coffee gum for maintaining oil and coffee flavouring constituents. The soluble dry coffee gum is preferably blended with the roasted coffee prior to grinding of the latter. During the blending and particularly during the grinding of the blend, the dry soluble gum particles serve as absorbents for oil and coffee flavouring constituents.
U.S. Pat. No. 8,603,562 relates to coffee-derived surfactants which are provided by alkaline catalysed transesterification reactions of sugars and coffee oil. The coffee-derived surfactants are especially useful in the production of coffee oil emulsions for use with, or incorporation into, coffee products. The coffee-derived surfactants increase the incorporation of coffee oil into the coffee products with reduced formation of so-called “coffee slicks”. Incorporation of the coffee in the coffee product maintains some of the coffee flavour in the final product.
Enzymes have been used to increase the quality or the yield in a number of documents. Thus, U.S. Pat. No. 5,714,183 discloses a method for hydrolysing galactomannans in a liquid coffee extract to reduce or eliminate formation of a gel during freezing of the extract. The liquid coffee extract is produced by hydrolysing the extract with an immobilized beta-mannanase at a sufficient temperature and for a sufficient time to remove galactomannans from the extract and form a final product which is substantially free of beta-mannanase.
U.S. Pat. No. 4,983,408 discloses a method for producing an improved yield of coffee extract which comprises: (1) pre-treating an aqueous mixture of coffee grounds with steam in a closed vessel under pressure at a temperature in excess of 200° C.; (2) maintaining said temperature and pressure for a period of from about 1-10 minutes; (3) instantaneously exposing the contents of said vessel to the atmosphere without cooling so as to bring said contents to atmospheric levels; and (4) treating the slurry thus obtained with a member selected from the group consisting of a hydrolytic enzyme and a mixture of hydrolytic enzymes. The process produces thermal damage by-products and the yield is suboptimal.
Membrane based methods have been suggested in EP1844661, which discloses a method to produce a soluble coffee extract, said method comprising: (1) finely wet milling roasted coffee solids to form a coffee slurry containing coffee solids; (2) treating the coffee slurry with an effective amount of an enzyme in the form of a stabilized enzyme composition at a temperature and for a time sufficient to hydrolyse the coffee solids to form a soluble coffee extract material, wherein the stabilized enzyme composition comprises the enzyme and an effective amount of a coffee-derived material to stabilize the enzyme; and (3) separating the soluble coffee extract material into a retentate and a permeate, wherein the permeate comprises the soluble coffee extract.
EP 1.745.702 also relates to a membrane based method, wherein a coffee extract is produced by finely wet-milling coffee beans or ground coffee or pre-extracted coffee grounds with hydrolase enzymes, preferably carbohydrase or protease enzymes, e.g. glucanases and mannanases, or mixtures thereof, the mixtures preferably comprising mannanase, cellulase and protease enzymes, and wherein the enzymes are retained in the reaction zone, via the use of a membrane device, so that the finished extract is essentially devoid of enzyme, oil or particulates and the enzyme(s) can eventually be re-used. The process results in a reaction zone in which only a small amount of 5-hydroxymethyl furfural (5-HMF) is present, because 5-HMF penetrates the membrane and thus does not inhibit the enzyme activity.
In EP 1,745,702 it is realized that 5-HMF may impart an undesirable winey or hay-like taste (page 229 of Coffee Flavour Chemistry, Ivon Flament, Wiley 2002). The present inventors have discovered that 5-HMF and other thermal decomposition products also seriously inhibit the activity of many enzymes. However, the suggestion of using a membrane-based process adds to the complexity of the over-all process and to the investment needed in the process. The present invention suggests a process design which avoids or reduces the presence of 5-hydroxymethyl furfural (5-HMF) and other undesired decomposition products during the action of the hydrolysing enzymes, thereby improving the reaction conditions for the enzymes.